It is known to use chromatic confocal techniques in optical height or distance sensors. As described in U.S. Patent Application Publication No. US2006/0109483 A1, which is hereby incorporated herein by reference in its entirety, an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the surface height or distance relative to the focusing element determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole or the end of an optical fiber. Upon reflection from the surface and passing back through the optical system to the in/out fiber, only the wavelength that is well-focused on the surface is well-focused on the fiber. All of the other wavelengths are poorly focused on the fiber, and so will not couple much power into the fiber. Therefore, for the light returned through the fiber, the signal level will be greatest for the wavelength corresponding to the surface height or distance to the surface. A spectrometer type detector measures the signal level for each wavelength, in order to determine the surface height.
Certain manufacturers refer to practical and compact systems that operate as described above, and that are suitable for chromatic confocal ranging in an industrial setting, as chromatic point sensors. A compact chromatically-dispersive optical assembly that is used with such systems is referred to as an “optical pen”, or a “pen”. The optical pen is connected through an optical fiber to an electronic portion of the chromatic point sensor which transmits light through the fiber to be output from the optical pen and which provides a spectrometer that detects and analyzes the returned light. The returned light forms a wavelength-dispersed intensity profile received by the spectrometer's detector array. Pixel data corresponding to the wavelength-dispersed intensity profile is analyzed to determine the “dominant wavelength position coordinate” for the intensity profile, and the resulting pixel coordinate is used with a lookup table to determine the distance to the surface.
An important issue with chromatic point sensors is the stability of their components relative to their calibration. Chromatic point sensors provide very high resolution and accuracy (e.g., sub-micron resolution and accuracy) based on distance calibration data that correlates known measurement distances with the resulting dominant wavelength position coordinate along the array. At the level of resolution and accuracy provided by chromatic point sensors, component behavior inevitably drifts relative to the behavior provided at the time of calibration, resulting in measurement errors. Known methods of recalibration generally require equipment and/or a level expertise that is impractical for end-users to provide. Thus, if the measurement accuracy degrades, or if a user desires to replace a specific component of the chromatic point sensor (such as the optical pen), the entire unit may need to be sent back to the factory for recalibration.
European Patent EP 1647799 to Michelt (the '799 Patent), which is hereby incorporated herein by reference in its entirety, discloses a method for separately calibrating a chromatic point sensor pen and a chromatic point sensor spectrograph that are to be used in combination. According to the '799 Patent, first calibration factors are determined for the pen by using a calibrated spectrometer to establish a certain wavelength and determining the certain distance from the measuring head to a reference surface for which the maximum intensity arises for that certain wavelength. Second calibration factors are determined for the chromatic point sensor spectrograph by determining which output signals are produced by the spectrograph when one supplies it with light of that certain wavelength. However, for various reasons, the method of the '799 Patent has not proven satisfactory in commercial practice. Providing improved, simplified, and/or more reliable recalibration for chromatic point sensors is desirable.